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LoongArch: Do not save/restore percpu base register in rethook trampoline

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The rethook trampoline saves $r21 ($u0), the percpu base, into its frame
at entry and restores it at exit. Inbetween rethook_trampoline_handler()
may schedule via preempt_enable_notrace().

If the task migrates to another CPU, the frame's $r21 holds the old
CPU's percpu base, and restoring it poisons $r21 on the new CPU. Until
the next user->kernel transition heals $r21, all this_cpu_*() accesses
(runqueues, RCU per-CPU data, timer tick programming, FPU ownership)
hit the wrong CPU's percpu area.

Under kretprobe-heavy preemptible load this can corrupt scheduler and
timer state: scheduling-while-atomic splats, wrong-CPU RCU warnings,
WARN_ON_ONCE(rq != this_rq()) in nohz_balance_exit_idle(), and CPUs
parking in the idle loop with the constant timer never re-armed (hard
lockup). Reproduces on a Loongson-3A6000 with kretprobes on VFS paths
plus heavy file churn (OS install / unsquashfs).

By convention $r21 always holds the current CPU's percpu base in kernel
mode: SAVE_SOME() at exception entry reloads it only when coming from
user mode, and RESTORE_SOME() restores it only when returning to user
mode; the context-switch path never writes it. Therefore the live $r21
at trampoline exit is already correct, and nothing inbetween can change
it legitimately (kernel C code cannot write a global register variable).
The same flaw existed even in the pre-rethook kretprobe trampoline since
v6.3; it was carried over when rethook replaced it. Drop both the save
and the restore here. Drop the restore is enough to solve the issue, and
drop the save is to keep the code tidy and no need to clear it.

Cc: stable@vger.kernel.org # v6.3+
Fixes: 3f55368600 ("LoongArch: Add kretprobes support")
Assisted-by: Kimi:Kimi-K3 # debug and root-cause analysis
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
This commit is contained in:
Wentao Guan 2026-09-04 21:44:43 +08:00 committed by Huacai Chen
parent 3e1b64bd8c
commit c3f2feace5
1 changed files with 0 additions and 2 deletions
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2
arch/loongarch/kernel/rethook_trampoline.S
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@ -24,7 +24,6 @@
cfi_st t6, PT_R18
cfi_st t7, PT_R19
cfi_st t8, PT_R20
cfi_st u0, PT_R21
cfi_st fp, PT_R22
cfi_st s0, PT_R23
cfi_st s1, PT_R24
@ -59,7 +58,6 @@
cfi_ld t6, PT_R18
cfi_ld t7, PT_R19
cfi_ld t8, PT_R20
cfi_ld u0, PT_R21
cfi_ld fp, PT_R22
cfi_ld s0, PT_R23
cfi_ld s1, PT_R24